If all vehicles had been equipped with autobrake that worked as well as the systems studied, there would have been at least 700,000 fewer police-reported rear-end crashes in 2013. That number represents 13 percent of police-reported crashes overall.

"The success of front crash prevention represents a big step toward safer roads," says David Zuby, IIHS chief research officer. "As this technology becomes more widespread, we can expect to see noticeably fewer rear-end crashes. The same goes for the whiplash injuries that often result from these crashes and can cause a lot of pain and lost productivity."

Front crash prevention is steadily becoming more prevalent, but in most cases it is offered as optional equipment. That may soon change, however. In September, the National Highway Traffic Safety Administration and IIHS announced an agreement in principle with automakers to make autobrake standard on all models.

The new IIHS findings are in line with earlier research by HLDI based on insurance claim rates (see "Evidence continues to mount in favor of front crash prevention," Aug. 26, 2015, and Status Report special issue: crash avoidance, July 3, 2012).

Using police reports allows researchers to identify front-to-rear crashes in order to gauge front crash prevention systems' effectiveness specifically for the type of collision they are designed to address.

For the study, researchers looked at police-reported rear-end crashes in 22 states during 2010-14 involving Acura, Honda, Mercedes-Benz, Subaru and Volvo vehicles with optional front crash prevention. The crash rates of vehicles equipped with the technology were compared with the crash rates of the same models without front crash prevention. Individual vehicles with the technology were identified using trim level information or, in some cases, lists of vehicle identification numbers supplied by the manufacturers.

A separate analysis of City Safety, Volvo's standard low-speed autobrake system, was conducted by comparing the S60 with other midsize luxury four-door cars and the XC60 with other midsize luxury SUVs. Unlike the City Safety-equipped Volvos, none of the comparison vehicles had standard front crash prevention.

Only rear-end crashes in which the study and comparison models struck other vehicles were considered. Crashes in which those vehicles were struck from behind but didn't strike a vehicle in front were left out, since front crash prevention wouldn't be expected to prevent them.

Information from HLDI's database was used to control for factors that might have affected crash rates, including the vehicle's garaging location and driver characteristics.

The analyses show that forward collision warning alone reduces rear-end crashes by 23 percent, while forward collision warning with autobrake reduces them by 39 percent. The reduction for City Safety is 41 percent.

The study also shows that autobrake reduces injuries. The rate of rear-end crashes with injuries decreases by 42 percent with forward collision warning with autobrake and 47 percent with City Safety. Forward collision warning alone is associated with a 6 percent decrease in rear-end injury crashes, though that finding isn't statistically significant.

"Even when a crash isn't avoided, systems that have autobrake have a good chance of preventing injuries by reducing the impact speed," says Jessica Cicchino, the study's author and the Institute's vice president for research. "Still, it's surprising that forward collision warning didn't show more of an injury benefit, given that HLDI has found big reductions in injury claims with the feature."

One difficulty in studying optional front crash prevention systems is that they often are packaged with other crash avoidance technologies. For example, all of the study vehicles except for some Honda Accords and most of the City Safety-equipped Volvos had adaptive cruise control. Adaptive cruise control works like regular cruise control but uses sensors to track the vehicle in front to maintain a safe following distance.

It is possible that some of the observed benefit for front crash prevention systems in avoiding rear-end collisions is actually a result of adaptive cruise control. However, unlike front crash prevention, drivers must activate adaptive cruise control every time they use it, and the feature generally isn't used for all types of driving.

Lane departure warning was packaged with front crash prevention on the Hondas, Subarus and some Volvos included in the study, but it is unlikely to have affected rear-end crashes.

Cicchino performed an additional analysis of City Safety vehicles to see how the effect of the system varied depending on a road's speed limit. The study vehicles had a version of City Safety that works at speeds up to 19 mph. (A newer version works at speeds up to 30 mph.)

Despite its speed limitation, City Safety had the biggest effect on roads with speed limits of 40-45 mph. The equipped Volvos rear-ended other vehicles 54 percent less frequently than comparable vehicles on those roads. The reduction was 39 percent on roads with speed limits of 35 mph or less and 25 percent on roads with speed limits of 50 mph or higher.

"At first blush it's surprising that this low-speed system was most effective on 40-45 mph roads," Cicchino says. "However, these roads tend to have many traffic lights, which reduce actual travel speeds in places. In addition, City Safety can come into play whenever there is congestion on a higher-speed road."